Soil Hydraulic Properties Research Articles

Analysis of influence of air-entry values to unsaturated soil properties

Soil slope instability is a grave concern in regions with high annual precipitation, especially in tropical countries. The impact of soil instability on the economies of these countries is severe since it usually causes fatalities and property loss. The research and development of the systems to reduce the impact of soil slope failures by predicting slope failure, monitoring the slope, and early failure warning has made it possible to implement safety measures. Despite the considerable effort to implement these measures, the factors contributing to soil slope instability are complex and have not been explored extensively. The air-entry value (AEV), which is the matric suction, is a critical factor in predicting the risk of slope instability. This paper will analyze the interaction of the basic soil properties and the soil hydraulic properties, also known as AEV. The researchers collected natural undisturbed soil samples from several locations to determine their AEVs which obtained by plate pressure extractor apparatus. The basic soil properties, namely the natural moisture content, particle size distribution and bulk density were determined in the laboratory, and the values were used to determine the correlation between the basic soil properties and the soil AEV. The results show that the soil with higher coefficient of curvature and percentage of sand have lower air-entry values, which mean they could experience disastrous slope failure during heavy rainfalls. This strong correlation underscores the significance of soil hydraulic properties as the critical factor in soil slope instability.

Soil hydraulic properties and pore dynamics under different tillage and irrigated crop sequences

Soil hydraulic properties and pore continuity are important parameters of soil quality and may differ among tillage systems, crop rotations, and change over time. However, there are some contrasting results, depending on soil type, climate and cultivation history on the spatial and temporal variations. The objective of this study was to determine spatio-temporal dynamics of soil hydraulic properties and pore characteristics (i.e., pore volume, effective porosity, and continuity) on a silt loam long-term tillage field experiment, in Agramunt, NE Spain, during two cropping years. Undisturbed soil samples were used to determine soil water retention, θ(Ψ), and soil hydraulic conductivity, K(Ψ), curves in two different tillage systems (intensive tillage, IT vs no-tillage, NT), two crop sequences (short fallow-maize, FM vs legume-maize, LM) and two positions (within the row of crops, W-row vs between the rows of crops, B-row). The results revealed that LM had greater specific hydraulic conductivity, Kpc in its macroporosity (>1000 µm) than FM due to greater number of effective pores, Npc and effective porosity, εpc. Soil water content, θ was greater under IT than under NT at higher soil water matric potential, Ψ (≥-3 cm H2O), whilst the opposite was observed at lower Ψ (≤ − 50 cm H2O). Long-term NT showed greater hydraulic conductivity, K at higher Ψ (≥ − 10 cm H2O) than IT, and no difference at lower Ψ. Although IT had greater pore volume, ϕpc than NT in the macroporosity and coarse mesoporosity (1000–60 µm) pore size classes, NT had two times greater Kpc than IT due to increased Npc, εpc, and pore continuity, Cwpc. K(Ψ) and pore characteristics showed spatial variations (W-row vs B-row). W-row had significantly greater K at higher Ψ (≥ − 3 cm H2O) than B-row. Temporal dynamics of soil hydraulic properties and pore characteristics were not evident under IT during crop succession. This study shows that LM increases specific hydraulic conductivity of soil macroporosity by increasing the number of effective macropores and the effective porosity. In a Mediterranean climate, this may improve the hydrological functions of agricultural soil and associated crop yield. Further, long-term NT formed a stable number of effective macropores and coarse mesopores, and showed a greater pore continuity in coarse and fine mesopores, resulting in improved soil water flux.

Richards Equation at the Hillslope Scale: Can We Resolve the Heterogeneity of Soil Hydraulic Material Properties?

AbstractProcess‐based modeling of soil water movement with the Richards equation requires the description of soil hydraulic material properties, which are highly uncertain and heterogeneous at all scales. This limits the applicability of the Richards equation at larger scales beyond the patch scale. The experimental capabilities of the three hillslopes of the Landscape Evolution Observatory (LEO) at Biosphere 2 provide a unique opportunity to observe the heterogeneity of hydraulic material properties at the hillslope scale. We performed a gravity flow experiment where through constant irrigation the water content increases until the hydraulic conductivity matches the irrigation flux above. The dense water content sensor network at LEO then allows mapping of the heterogeneity of hydraulic conductivity at a meter scale resolution. The experiment revealed spatial structures within the hillslopes, mainly a vertical trend with the lowest hydraulic conductivity close to the surface. However, the variation between neighboring sensors is high, showing that the heterogeneity cannot be fully resolved even at LEO. By representing the heterogeneity in models through Miller scaling we showed the impact on hillslope discharge. For the hillslope with the smallest heterogeneity, representing the dominant structures was sufficient. However, for the two hillslopes with the larger overall heterogeneity, adding further details of the local heterogeneity did impact the discharge further. This highlights the limitations of the Richards equation, which requires the heterogeneous field of material properties, at the hillslope scale and shows the relevance to improving our understanding of effective parameters to be able to apply the process‐based model to larger scales.

Effect of hydrogel on corn growth, water use efficiency, and soil properties in a semi-arid region

Most countries, particularly those in arid and semi-arid regions, are grappling with important agricultural production issues that influenced by rapid population growth and limited water resources. The application of soil additives to improve soil properties and water usage productivity has sparked the most attention, particularly in arid and semi-arid environments. The goal of this study was to assess the effect of hydrogel on soil physical properties and plant growth parameters using sandy and silty clay loam soils. Laboratory experiments were implemented using ten different concentrations of hydrogel based on the percentage of the added hydrogel (0, 0.02, 0.04, 0.07, 0.09, 0.11, 0.16, 0.22, 0.27 and 0.33% (w/w) hydrogel/soil) to assess its effect on soil physical and hydraulic properties. Moreover, a greenhouse pot experiment was conducted using four hydrogel concentrations of 0, 0.25, 0.5, and 1% (w/w) on Zea mays as a model plant to assess its effect on plant growth parameters. Results revealed that the greatest improvement in soil aggregate percentage was 35% with 0.27% hydrogel concentration whereas, 0.33% hydrogel concentration increased the soil available water by 49%. Moreover, water use efficiency was increased from 13% to 41% for sandy soil and from 35% to 67% for silty clay loam soil. In addition, when compared to the control, both soils' water use efficiency and corn growth rose. In agriculture, hydrogel can improve soil physical properties while also boosting water use efficiency and plant development parameters in dry and semi-arid areas.

Simulating water and nitrogen runoff with APSIM

To determine the impact of potential reductions of terrain-targeted nitrogen (N) fertilisation rates on N losses a simulation study was carried out using the Agricultural Production Systems Simulator (APSIM). To simulate N runoff a simple approach was used, in which runoff is based on the N concentration in the soil solution and an extraction coefficient. Firstly, APSIM parameters that have the largest effect on runoff of water and N were determined for terrains with different slopes for a poorly drained silt loam. A sensitivity analysis was then conducted to assess the effect of soil hydraulic properties and soil organic carbon content on runoff losses. Finally, APSIM was set up to simulate pasture production and water and N dynamics (including pasture N uptake, leaching and N runoff) for a farm on rolling hills in South Canterbury, New Zealand. Two different fertilisation approaches were used, either scheduled or based on the aboveground N concentration of the pasture. For the poorly drained silt loam, the rainfall intensity and the surface conductance had the highest effect on the amount of water lost by runoff. Soil hydraulic conductivity at saturation and field capacity, as well as plant available water content also controlled runoff of water and N, while the organic carbon content of the topsoil had less effect on N runoff. Both the extraction coefficient and the depth considered to exchange N with the runoff water affected the amount of N lost via runoff. Using the aboveground pasture N concentration prior to fertilisation had positive effects on pasture yield and reduced N runoff losses.

Review of Ground Penetrating Radar Applications for Water Dynamics Studies in Unsaturated Zone

For water dynamics investigation in unsaturated (vadose) zones, ground penetrating radar is a popular hydro-geophysical method because it is non-invasive for soil, has high resolution and the results have a direct link with water content. Soil water content and soil hydraulic properties are two key factors for describing the water dynamics in vadose zones. There has been tremendous progress in soil water content and soil hydraulic properties estimation with ground penetrating radar. The purpose of this paper is to provide an overview of the application of ground penetrating radar for soil water dynamics studies. This paper first summarizes various methods for the determination of soil water content. including traditional methods in the surveys of surface ground penetrating radar, borehole ground penetrating radar, and off-ground ground penetrating radar, as well as relatively new methods, such as full waveform inversion, the average envelope amplitude method, and the frequency shift method. This paper further provides a review for estimating soil hydraulic properties with GPR according to the types of ground penetrating radar data. We hope that this review can provide a reference for the application of ground penetrating radar in soil water dynamics studies in the future.

The response of soil physical quality parameters to a perennial grain crop

Soil physical quality is paramount for root growth, water and air movement, and for its subsequent effects on chemical and biological processes in the soil. Management practices and their legacies can impact soil physical quality, and perennial grain cropping has been proposed as a solution to maintain or improve soil physical quality in agroecosystems due to their provision of year-round ground cover and increased root growth. An alfalfa-brome perennial forage crop, a perennial rye crop (Secale cereale L. x S. montanum L.), and an annual rye crop (S. cereale L.) were evaluated at two sites in Central Alberta with contrasting management histories (Edmonton and Breton) over 3 years to determine the effects on soil physical and hydraulic properties. Compared to the annual crop, the perennial forage crop reduced the bulk density of the uppermost soil depth sampled (5–10 cm depth increment) (p < 0.05) at the Edmonton site and increased soil macroporosity (p < 0.05) and pore connectivity (p < 0.05) in the deeper subsurface soil layer (25–30 cm depth increment) at both sites. While moderate improvements in soil physical and hydraulic properties manifested under the perennial rye crop when compared to the annual rye crop, they did not do so to the extent of the perennial forage crop. We attribute this to the inclusion of tap-rooted alfalfa in the perennial forage, and the overarching beneficial influence of root mass density on soil properties. Root mass density from highest to lowest consistently ranked as perennial forage > perennial rye > annual rye for both sites. Root mass density was negatively correlated with bulk density at both Breton (r = −0.77, p < 0.05) and Edmonton (r = −0.69, p < 0.05) sites. Furthermore, at Breton, root mass density positively correlated with macroporosity (r = 0.88, p < 0.01). Notably, the perennial rye crop enhanced soil carbon mass density relative to the annual rye crop in the clayey topsoil of the Edmonton site (p < 0.05), but treatment effects were muted at the Breton site due to the influence of previous land use. Despite moderate improvements in soil physical quality, our results suggest that 3 years of perennial rye monocropping falls short of the major improvements seen under a perennial alfalfa-brome forage crop over the same timeframe.

Investigating the Spatial Structure of Soil Hydraulic Properties in a Long-Term Field Experiment Using the BEST Methodology

Understanding the spatial structure of soil properties at field scale and introducing this information into appropriate data analysis methods can help in detecting the effects of different soil management practices and in supporting precision agriculture applications. The objectives of this study were: (i) assessing the spatial structure of soil physical and hydraulic properties in a long-term field experiment; (ii) defining a set of spatial indicators for gaining an integrated view of the studied system. In seventy-two georeferenced locations, soil bulk density (BD), initial volumetric soil water content (θi) and cumulative infiltration curve as function of the time (I(t)) were measured. The soil water retention curve (θ(h)) and the hydraulic conductivity function (K(h)) were then estimated using the Beerkan Estimation of Soil Transfer parameters (BEST) methodology. The volumetric soil water contents at soil matrix (h = −10 cm), field capacity (h = −100 cm) and wilting point (h = −15,300 cm) were considered. In addition, a set of capacitive indicators—plant available water capacity (PAWCe), soil macroporosity (PMACe), air capacity (ACe) and relative field capacity (RFCe)—were computed. The data were first analyzed for overall spatial dependence and then processed through variography for structural analysis and subsequent spatial interpolation. Cross-correlation analysis allowed for assessing the spatial relationships between selected physical and hydraulic properties. On average, optimal soil physical quality conditions were recorded; only PMACe values were indicative of non-optimal conditions, whereas mean values of all the other indicators (BD, Ks, PAWCe, ACe, RFCe) fell within optimal ranges. The exponential model was found to be the best function to describe the spatial variability of all the considered variables, except ACe. A good spatial dependence was found for most of the investigated variables and only BD, ACe and Ks showed a moderate autocorrelation. Ks was confirmed to be characterized by a relatively high spatial variability, and thus, to require a more intensive spatial sampling. An inverse spatial cross-correlation was observed between BD and Ks up to a distance of 10 m; significant cross-correlations were also recorded between Ks and PMACe and ACe. This result seems to suggest the possibility to use these soil physical quality indicators as covariates in predictive multivariate approaches.

Fire-induced changes in soil properties depend on age and type of forests

Abstract Wildfires affect different physical, chemical, and hydraulic soil properties, and the magnitude of their effects varies depending on intrinsic soil properties and wildfire characteristics. The objectives of this study are: to estimate the impact of heating temperature (50–900°C) on the properties of sandy soil (Arenosol) taken in 1) coniferous forests (Scots pine Pinus sylvestris) of different ages (30 and 100 years); and 2) coniferous (Scots pine Pinus sylvestris) and deciduous (alder Alnus glutinosa) forests of the same age (30 years). The forests are located in the central part of the Borská nížina lowland (western Slovakia), and the properties treated were soil organic carbon content (SOC), pH, and soil water repellency (measured in terms of water drop penetration time, WDPT). It was found that the impact of heating temperature on the properties of sandy soil is great and depends on both the age and type of forest. The SOC value decreased unevenly with temperature in all three soils, and it was higher in the 30-year-old deciduous forest soil than in the 30-year-old coniferous forest soil. The value of pH increased monotonously with temperature from 200 °C, and it was higher in 30-year-old coniferous forest soil than in the 100-year-old coniferous forest soil. SOC and WDPT in the 100-year-old coniferous forest soil were higher than SOC and WDPT in the 30-year-old coniferous forest soil. Results obtained (decrease in SOC, disappearance of SWR after heating to 400 °C, and increase in pH from heating temperature 200 °C) bring important information for post-fire vegetation restoration and post-fire management of Central European forests established on sandy soil.

Effects of prescribed fire on topsoil properties: a small-scale straw burning experiment

Abstract A grassland was burned to investigate how a short prescribed fire affected soil physical and hydraulic properties, soil water balance, and emergent vegetation. Three years before the experiment at Řisuty, Czech Republic, the grassland was re-established on arable soil. At the experimental site there is a weather station and sensors measuring soil temperature and moisture at three different depths. The 5 m × 5 m burned plot was compared to a nearby unburned reference location. The loamy Cambisol soil was not water-repellent. 250 m2 of sun-dried grass was raked and burned at the burned plot. The fire lasted approximately 15-minute and reached 700 °C. Soil samples were taken immediately after the fire and weekly to monthly thereafter to quantify organic carbon content, soil structure stability, hydraulic conductivity, bulk density, and texture. According to the research results, it appears that temporary burning improved the hydraulic properties of the topsoil. The fire plot’s infiltration capacity was increased, and soil water content was higher than the control plot throughout the year, providing suitable habitat for colonizing vegetation. The results suggest that small-scale controlled biomass burning can be risk-free to the soil ecosystem and may even temporarily improve the hydraulic properties of the upper soil layer.

Long-term analysis of soil water regime and nitrate dynamics at agricultural experimental site: Field-scale monitoring and numerical modeling using HYDRUS-1D

Intensive agricultural practices increase agrochemical pollution, particularly nitrogen (N) based fertilizers, which present an environmental risk. This study aims to evaluate long-term (2009–2020) data on soil water regime and nitrate dynamics at an agricultural experimental site on fine-textured soils and to better understand the implications of N management in relation to groundwater pollution. The field site is located in the Biđ field (eastern Croatia), in the proximity of the Sava river. Zero-tension lysimeters were installed at six selected locations. Lysimeters were used to monitor the water regime, i.e., outflows in which nitrate concentration was measured, while additional soil-water samples were collected via 4 and 15-meter-deep monitoring wells. Soil hydraulic parameters were estimated by combining the laboratory measurements, and estimation in RETC software. Water regime and nitrate leaching in lysimeters were simulated using HYDRUS-1D for each year to allow crop rotation and to evaluate their effects individually. The HYDRUS-1D model successfully reproduced lysimeter outflows and nitrate dynamics, which was confirmed with high R2 values (water: 93% above 0.7, and nitrate: 73% above 0.7) indicating the good performance of the model simulating nitrification chain reactions. Principal component analysis (PCA) was performed to identify the relationships among all soil properties and environmental characteristics. The results showed the complex interaction of soil hydraulic properties, precipitation patterns, plant uptake, and N application. All locations have a decreasing trend of nitrate leaching over the investigation period. Most of the lysimeter outflows and elevated nitrate concentrations were connected to the wet period of the year when the soil was saturated, and evapotranspiration was low. The results of this study show that it is important to optimize N fertilizer applications for each particular environmental condition to reduce nitrate loss. The study indicates the importance of long-term field studies, key for agro-hydrological modeling and the improvement of agricultural practices.

Micrometeorological and Hydraulic Properties of an Urban Green Space on a Warm Summer Day in a Mediterranean City (Attica–Greece)

Urban Green Spaces (UGSs) are considered the most effective tool to mitigate Urban Heat Islands (UHIs). The optical properties of the materials and the vegetation types of the UGSs affect their surface temperatures, directly influencing their cooling ability. The hydraulic properties of urban soils are also affected by the vegetation coverage. The aim of this study is to investigate the temperature and reflected radiation (albedo) differences between UGS’s elements, around noon on a warm summer day, in Greece. The results indicate that green elements have smaller surface temperatures and higher reflectance compared to the artificial or the dry bare soil, presenting differences with the direct air temperature (measured above the surfaces with unshielded thermometers) −5.5 °C (shrubs), −3.8 °C (grass), +7.8 °C or +8.7 °C (paved surfaces inside or outside the UGS), +10.8 °C (dry bare soil), +12.2 °C (concrete) and +12.5 °C (asphalt), and albedo values 0.14 (grass and shrubs), 0.15 (dry bare soil), 0.27 (concrete), 0.21 (asphalt) and 0.20 (paved surfaces). The tree shades also produce great surface differences. The unsaturated hydraulic conductivity of the urban soil is greater than the surfaces covered with grass compared to the shrub-covered or bare soil, presenting values of 27.6, 10.8 and 11.4 mm h−1, respectively.

Inverse estimation of hydraulic parameters of soils with rock fragments

The application of inverse modeling to determine the hydraulic properties of layered soils with rock fragments (RF > 2 mm) is associated with complex challenges such as selecting suitable hydraulic functions and defining a strategy to optimize many parameters. These two issues were addressed in this study by performing field drainage experiments in layered soil profiles (three layers) with different particle size distributions and different percentages of RF. Water contents (θ) and pressure heads (h) were monitored at different soil depths during drainage experiments (h ≥ −200 cm). Data of θ and h for a drier range (h < −5000 cm) were determined with a dew point potentiometer on disturbed soil samples. The unimodal and bimodal van Genuchten-Mualem functions were evaluated, and their parameters were optimized using the Hydrus-1D “Inverse solution”. The RETC curve fitting software was used to estimate the second set of parameters of the bimodal function. The parameters for the uni- and bimodal models (15 and 27 parameters, respectively) were repeatedly optimized by fixing some parameters while estimating others. The unimodal van Genuchten-Mualem function provided a satisfactory fit of θ and h measurements only when either drainage or dew point potentiometer measurements were used. On the other hand, the bimodal van Genuchten-Mualem function provided a satisfactory fit of θ and h measurements when both sets of data (drainage and dew point potentiometer measurements) were used simultaneously. Furthermore, the optimized parameters were physically consistent except for a few high saturated hydraulic conductivity values. Therefore, bimodal functions should be considered to represent the hydraulic properties of soils with RF. The problem of many calibrated parameters when using bimodal functions can be managed using sequential optimization, which allowed us to estimate 27 parameters for layered soils with RF successfully.

The Impacts of Bio-Based and Synthetic Hydrogels on Soil Hydraulic Properties: A Review.

Soil hydraulic properties are important for the movement and distribution of water in agricultural soils. The ability of plants to easily extract water from soil can be limited by the texture and structure of the soil, and types of soil amendments applied to the soil. Superabsorbent polymers (hydrogels) have been researched as potential soil amendments that could help improve soil hydraulic properties and make water more available to crops, especially in their critical growing stages. However, a lack of a comprehensive literature review on the impacts of hydrogels on soil hydraulic properties makes it difficult to recommend specific types of hydrogels that positively impact soil hydraulic properties. In addition, findings from previous research suggest contrasting effects of hydrogels on soil hydraulic properties. This review surveys the published literature from 2000 to 2020 and: (i) synthesizes the impacts of bio-based and synthetic hydrogels on soil hydraulic properties (i.e., water retention, soil hydraulic conductivity, soil water infiltration, and evaporation); (ii) critically discusses the link between the source of the bio-based and synthetic hydrogels and their impacts as soil amendments; and (iii) identifies potential research directions. Both synthetic and bio-based hydrogels increased water retention in soil compared to unamended soil with decreasing soil water pressure head. The application of bio-based and synthetic hydrogels both decreased saturated hydraulic conductivity, reduced infiltration, and decreased soil evaporation. Hybrid hydrogels (i.e., a blend of bio-based and synthetic backbone materials) may be needed to prolong the benefit of repeated water absorption in soil for the duration of the crop growing season.

Variation in Hydraulic Properties of Forest Soils in Temperate Climate Zones

The structure of forests in temperate climates has been changing to ensure the resilience of trees. This change affects the local water balance. Knowledge of soil hydraulic properties (SHP) is essential to assess the water cycle in ecosystems. There is little knowledge about the impact of tree species on SHP and the water balance. Based on a compilation of 539 related studies we aimed at identifying the effects of tree species and age on SHP in temperate climates. However, most studies concentrated on soil biogeochemical properties, whereas only 256 studies focused on SHP. The literature presents no standard methods for assessing SHP and there is no knowledge of their variations in forests. We present a systematic overview of the current state of knowledge on variations in SHP based on forest type in temperate climates. We identify the gaps and weaknesses in the literature and the difficulties of evaluating the reviewed studies. More studies following standardised methodologies are needed to create a robust database for each forest type and soil texture. It would improve the assessment of the forest water balance through calibrated plot/site-scale process models. Such a database does not yet exist, but it would greatly improve the management and development of future forest ecosystems.

Pore Size Distribution Derived from Soil–Water Retention Characteristic Curve as Affected by Tillage Intensity

Tillage practices can influence the pore size distribution (PSD) of the soil, affecting soil physical and hydraulic properties as well as processes that are essential for plant growth, soil hydrology, environmental studies and modeling. A study was conducted to assess the effect of no-tillage (NT) and conventional tillage (CT) on PSD derived from soil–water retention curves (SWRCs) using the van Genuchten’s equation (vG) at 0–15 cm and 15–30 cm depths in a sandy loam soil. Values of PSD or slopes (C(h)) were calculated from the SWRCs by differentiating the vG equation. Soil water retention curves under both tillage systems and within two depths were determined using the evaporation HYPROP method. The vG equation was well fitted to measured soil water retention data. The diameter (D) of soil pores retaining water at various matric suctions (|h|) of water in soils was calculated by the capillary equation. A significant effect of tillage on soil PSD was observed in the macro-pore (D &gt; 1000 μm, at |h| &lt; 3 hPa) and meso-pore (D between 10 and 1000 μm, at |h| between 300 and 3 hPa) size classes, while the micro-pores size class (D &lt; 10 μm, at |h| &gt; 300 hPa) was unaffected at the 0–15 and 15–30 cm depths. Larger values of C(h) or PSD in CT were associated with greater soil loosening induced by the CT operations and greater proportion of large pores (structural porosity) occurred in soils under CT compared to soils under NT. Macro-pore and meso-pore proportions were significantly greater in soils under CT than in soils under NT within both soil depths. The hydraulic parameters of the vG equation and its derivative function can be used to compare soil–water retention curves and pore size distributions between soils under untilled and tilled conditions.

Stochastic Merging of Soil Hydraulic Properties for Vadose Zone Hydrological Modeling

Soil hydraulic properties (SHPs) are commonly determined in soil samples with replicas. Whether these replicas are taken at a same location to represent a specific point or at several locations to represent a larger area, the results should be merged into a final data set to be used in modeling. For this data set to be representative, standard errors and a correlation matrix must be considered in the merging process. We present a method to perform this merging and give an example using stochastic realizations of van Genuchten-Mualem (VGM) parameters generated by Cholesky decomposition to merge the SHP and associated statistics into a merged parameter set. To do so, we used VGM parameters obtained at sample scale in three replicas from a Brazilian savanna soil through inverse modeling of laboratory evaporation experiments. The effectiveness and representativeness of the proposed methodology were evaluated by observing the frequency distribution of different levels of output, comparing individual and merged sample properties. The outputs include VGM parameters, retention and conductivity characteristics, and water balance components stochastically predicted by a hydrological model. The performed stochastic merging correctly represented the variability of the combined replicas, especially with respect to hydrological model outputs of soil water balance components. Using the mean hydraulic property parameter values to deterministically predict water balance components may yield values that are substantially different from the mean values of stochastic realizations. This suggests that the deterministic prediction using mean parameter values in vadose zone hydrological modeling may result in unrepresentative outputs.

A novel physical-empirical model linking shortwave infrared reflectance and soil water retention

Soil hydraulic properties, including soil water retention, control various terrestrial hydrological processes such as infiltration, runoff, and evapotranspiration. Measuring and monitoring these properties at large scales is challenging. Therefore, over the past decade, remote sensing has been investigated as a promising tool for large-scale mapping of soil hydraulic properties. Nonetheless, a solid physical relationship between soil hydraulic and spectral properties is still lacking. To close this knowledge gap, this study introduces a novel physical-empirical model that to our best knowledge for the first time connects the soil water retention curve (SWRC) to soil spectral reflectance. The new model has been developed based on the hypothesis that the capillary and adsorbed water components of the SWRC exhibit vastly different optical properties due to their distinct distribution within the soil porous system. The model was validated using soil water retention and reflectance measurements of 21 soils, vastly differing in physical and hydraulic properties. The model provides not only a new and accurate soil moisture-reflectance functional relationship, but also a potential means for retrieval of the soil water retention curve from spectral reflectance in the shortwave infrared domain.

Water Retention Characteristics of Mineral Forest Soils in Finland: Impacts for Modeling Soil Moisture

Soil hydraulic properties are central for soil quality and affect forest productivity and the impacts of climate change on forests. The water retention characteristics (WRC) of mineral forest soils in Finland are not well known, and practical tools to predict them for hydrological, biogeochemical and forest models are lacking. We statistically analyzed mineral forest soils WRC from over 130 sites in Finland, focusing on the humus layer and main root zone (0–19 cm depth). We showed that mineral forest soils can be grouped into five WRC classes that are well predictable from soil bulk density, organic matter content and clay fraction. However, the majority of the forest soils are hydrologically rather similar. We found that neither topsoil maps nor any combination of open geospatial data were able to predict WRC. Thus, in the absence of site-specific soil data, parameterizing WRC as a function of forest site fertility type was proposed. We demonstrated the approach in soil moisture modeling at a small forest headwater catchment and showed that the soil moisture response to weather conditions is jointly affected by WRC, stand attributes and topography. We showed that drought risks are highest for dense mature forests at nutrient-poor, coarse-textured sites and lower for young stands on peatlands and lowland herb-rich sites with groundwater influence. The results improve hydrological predictions for Finnish forests, and the open dataset can contribute to the larger synthesis and development of boreal forest soil pedo-transfer functions.

A novel double disc method to determine soil hydraulic properties from drainage experiments with tension gradients

Determination of the saturated hydraulic conductivity, Ks, and the water retention curve, θ(h), is of paramount importance to characterize the hydraulic behavior of the vadose zone. Given the van Genuchten hydraulic model, defined by the residual, θr, and saturated, θs, volumetric water content and the α and n parameters, this work presents a new laboratory procedure to estimate Ks, θs, n and α for a drainage process, αdr, from the inverse analysis of successive drainage steady-states curves generated by a tension-gradient between the surface and the base of a soil column. To this end, a double disc system, one connected a bubbling tower and placed at the soil surface and the second one placed under the soil core, was employed. The second disc was connected to an air-vacuum system. The experiment presented two parts: a first 1D downward infiltration at saturation on a dry soil column, followed by successive drainage steps. During the drainage process, the tension of the upper and lower discs varied between 0 and −5 cm, and from −5 to −100 cm, respectively. The soil sorptivity, S, and θs were calculated from the 1D transient infiltration measure, Ks was calculated by Darcy’s law, αdr and n were optimized from the inverse analysis of the steady-state curves under tension-gradient and α for a wetting process, αw, was calculated from previously obtained S, θs, Ks and n. Once Ks estimated, αdr and n were optimized by minimizing the Q=hb-hn objective function, where hb and hn are the experimental and calculated tensions at the base of the soil core. Given a αdr value, the optimimum n was computed as the value that provides a minimum Q. By repeating this process for a sequence of αdr, different Q-isolines were obtained, one for each hb value, which crossing-point corresponded to the actual αdr and n values. The method was tested on 2.5 cm high columns of four different synthetic soils. Next, it was applied on an experimental sand column of 5 cm height and on 2.5 cm high columns filled with sieved loam, clay loam and clay soil. The estimated αdr and n were compared with corresponding values measured in the same soils with the pressure plate technique and αw was contrasted with the corresponding value calculated with an empirical hysteresis model. The method, which was fast (from 1 to 2 h) and easy to implement for small-scale experiments, was successfully applied to soil samples 2.5 cm high and allowed to explore a range of soil tensions from 0 to −100 cm. Overall, accurate estimates of θs, Ks, αdr and n were obtained in both synthetic and experimental soils. A significant relationship was also obtained between αw estimated from S and the corresponding value calculated from the hysteresis model.